1. Field of the Invention
The present invention generally relates to methods and apparatus for expanding a tubular body in a wellbore. More specifically, the invention relates to methods and apparatus for forming a cased wellbore having an inner diameter that does not decrease with increasing depth within a formation.
2. Description of the Related Art
In well completion operations, a wellbore is formed to access hydrocarbon-bearing formations by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill support member, commonly known as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. Using apparatus known in the art, the casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. As more casing strings are set in the wellbore, the casing strings become progressively smaller in diameter in order to fit within the previous casing string. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
Decreasing the diameter of the wellbore produces undesirable consequences. Progressively decreasing the diameter of the casing strings with increasing depth within the wellbore limits the size of wellbore tools which are capable of being run into the wellbore. Furthermore, restricting the inner diameter of the casing strings limits the volume of hydrocarbon production which may flow to the surface from the formation.
Recently, methods and apparatus for expanding the diameter of casing strings within a wellbore have become feasible. As a result of expandable technology, the inner diameter of the cased wellbore does not decrease as sharply upon setting more casing strings within the wellbore as the inner diameter of the cased wellbore decreases when not using expandable technology. When using expandable casing strings to line a wellbore, the well is drilled to a first designated depth with a drill bit on a drill string, then the drill string is removed. A first string of casing is set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing is run into the drilled out portion of the wellbore at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second casing string is then expanded into contact with the existing first string of casing with an expander tool. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth.
An exemplary expander tool utilized to expand the second casing string into the first casing string is fluid powered and run into the wellbore on a working string. The hydraulic expander tool includes radially expandable members which, through fluid pressure, are urged outward radially from the body of the expander tool and into contact with the second casing string therearound. As sufficient pressure is generated on a piston surface behind these expansion members, the second casing string being acted upon by the expansion tool is expanded past its point of elastic deformation. In this manner, the inner and outer diameter of the expandable tubular is increased in the wellbore. By rotating the expander tool in the wellbore and/or moving the expander tool axially in the wellbore with the expansion member actuated, a tubular can be expanded into plastic deformation along a predetermined length in a wellbore.
The method of expanding the second casing string into the first casing string involves expansion of the second casing string past its elastic limit once located at the desired depth within the wellbore. Because a casing string is typically only capable of expansion to about 22–25% past its elastic limit, the amount of expansion of the casing string is limited when using this method. Expansion past about 22–25% of its original diameter may cause the casing string to fracture due to stress.
The advantage gained with using expander tools to expand expandable casing strings is the decreased annular space between the overlapping casing strings. Because the subsequent casing string is expanded into contact with the previous string of casing, the decrease in diameter of the wellbore is essentially the thickness of the subsequent casing string. However, even when using expandable technology, casing strings must still become progressively smaller in diameter in order to fit within the previous casing string.
Currently, monobore wells are being investigated to further limit the decrease in the inner diameter of the wellbore with increasing depth. Monobore wells would theoretically result when the wellbore is approximately the same diameter along its length, causing the path for fluid between the surface and the wellbore to remain consistent along the length of the wellbore and regardless of the depth of the well. With a monobore well, tools could be more easily run into the wellbore because the size of the tools which may travel through the wellbore would not be limited to the constricted inner diameter of casing strings of decreasing inner diameters. Theoretically, in the formation of a monobore well, a first casing string could be inserted into the wellbore. Thereafter, a second casing string of a smaller diameter than the first casing string could be inserted into the wellbore and expanded to approximately the same inner diameter as the first casing string.
Certain problems have arisen during the investigation of monobore wells. One problem relates to the expansion of the smaller casing string into the larger casing string to form a sealed connection therebetween where the first and second casing strings overlap. Forming a monobore well would involve first running the smaller casing string through the restricted inner diameter of the wellbore produced by the larger casing string, then expanding the smaller casing string to an inner diameter at least as large the smallest inner diameter of the larger casing string. This portion of the expansion of the smaller casing string likely would increase the inner diameter of the smaller casing string by the limit of 22–25%. To insert an even smaller casing string inside the smaller casing string to form a monobore well, the inner diameter of a lower portion of the smaller casing string would have to be enlarged to receive the even smaller casing string. In this way, expansion of the casing string to over 25% of its original diameter would be necessary, but not currently possible. Merely expanding the casing string past its elastic limit after passing the restricted inner diameter portion may not allow the casing string to expand to a large enough inner diameter to form a substantially monobore well, as the percentage which the casing string may expand past its elastic limit is limited by structural constraints of the casing string. Attempts to expand the casing string further than about 22–25% past its elastic limit may cause the casing string to fracture or may simply be impossible.
Another type of expansion is currently performed in the context of casing patches. A casing patch is a tubular body which is expanded into contact with the wellbore or casing within the wellbore to patch leaking paths existing in the wellbore or cased wellbore. To patch the leaking path within the casing or wellbore, a casing patch is often deformed so that the casing patch possesses a smaller inner diameter than the inner diameter of the existing casing or wellbore, then the casing patch is reformed to a larger inner diameter when the casing patch is located at the desired location for reformation of the casing patch. The reforming process is often performed by an expander cone. This method often leaves stress lines in the reformed casing patch where the corrugations originally existed, weakening the casing patch at the stress lines so that the casing patch is susceptible to leaking wellbore fluids into the casing patch due to the pressure exerted by wellbore fluids.
Utilizing the current methods of expanding a casing string or reforming a casing patch, the problems described above are evident when a casing string or casing patch must run through a restriction in the inner diameter of the wellbore, such as a restriction formed by a packer or a previously installed casing patch, and then expand to an inner diameter at least as large as the restriction once the casing string or casing patch is lowered below the restriction. When using a casing patch, merely reforming the casing patch may leave stress lines in the casing patch which may allow fluid leakage therethrough. When using a casing string, merely expanding the casing string past its elastic limit by 22–25% may not allow enough expansion to increase the inner diameter of the casing string to at least the inner diameter of the restriction.
There is, therefore, a need for a method for enlarging the inner diameter of a casing string or other tubular body by more than current methods allow without compromising the structural integrity of the casing string or tubular body. There is a further need for a method for expanding the inner diameter of a casing string or tubular body by a larger percentage than the percentage expansion allowed past the elastic limit after running the casing string or tubular body through a restricted inner diameter portion of the wellbore. There is yet a further need for a method of expanding a lower portion of the inner diameter of a casing string or tubular body further than the remaining portions of the casing string or tubular body without compromising the structural integrity of the lower portion of the casing string or tubular body.